Cellulose

Like human bone, plant cell walls are composed of fibrils laid down in a matrix, or "background" material. In a cell wall, the fibrils are cellulose microfibrils, and the matrix is composed of other polysaccharides and proteins. One of these matrix polysaccharides in cell walls is pectin, the substance that, when heated, forms a gel. Pectin is the substance that cooks use to make jellies and jams.

The arrangement of cellulose microfibrils within the polysaccharide and protein matrix imparts great strength to plant cell walls. The cell walls of plants perform several functions, each related to the rigidity of the cell wall. The cell wall protects the interior of the plant cell, but also allows the circulation of fluids within and around the cell wall. The cell wall also binds the plant cell to its neighbors. This binding creates the tough, rigid skeleton of the plant body. Cell walls are the reason why plants are erect and rigid. Some plants have a secondary cell wall laid over the primary cell wall. The secondary cell wall is composed of yet another polysaccharide called lignin. Lignin is found in trees. The presence of both primary and secondary cell walls makes the tree even more rigid, penetrable only with sharp axes.

Unlike the other components of the cell wall, which are synthesized in the plant's Golgi body (an organelle that manufactures, sorts, and transports different macromolecules within the cell), cellulose is synthesized on the surface of the plant cell. Embedded within the plant's plasma membrane is an enzyme, called cellulose synthetase, which synthesizes cellulose. As cellulose is synthesized, it spontaneously forms microfibrils that are deposited on the cell's surface. Because the cellulose synthetase enzyme is located in the plasma membrane, the new cellulose microfibrils are deposited under older cellulose microfibrils. Thus, the oldest cellulose microfibrils are outermost on the cell wall, while the newer microfibrils are innermost on the cell wall.

As the plant cell grows, it must expand to accommodate the growing cell volume. However, because cellulose is so rigid, it cannot stretch or flex to allow this growth. Instead, the microfibrils of cellulose slide past each other or separate from adjacent microfibrils. In this way, the cellwall is able to expand when the cell volume enlarges during growth.